Field of the Invention
The invention relates to a plasma spraying apparatus which transfers plasma-arc to an electrically conductive wire to thereby generate plasma flame, melts the wire into droplets, and sprays the droplets onto a target.
Description of the Related Art
FIG. 1 is a cross-sectional view of a conventional plasma spraying apparatus.
As illustrated in FIG. 1, the conventional plasma spraying apparatus 90 includes a first gas nozzle 91 defining a first gas path 91a, a second gas nozzle 92 disposed outside of the first gas nozzle 91 to define a second gas path 92a, a cathode 93 disposed substantially on central axes of both a nozzle opening 91b of the first gas nozzle 91 and a nozzle opening 92a of the second gas nozzle 92, a battery unit 94, and a wire guide hole 95 for introducing an electrically conductive wire W to be sprayed, into a vicinity of the nozzle opening 92a of the second gas nozzle 92.
The wire W is supplied obliquely of a central axis of the nozzle opening 92a and in front of the nozzle opening 92a through the wire guide hole 95. A first gas sprayed through the first gas path 91a is turned into plasma flame F by means of arc generated between the wire W indirectly electrically connected to an anode of the battery unit 94 through the second gas nozzle 92, and the cathode 93 electrically connected to a cathode of the battery unit 94. The thus generated plasma flame F melts the wire W into droplets D, and sprays the droplets D. The droplets D are further reduced in size and further accelerated by a second gas sprayed forwardly of the second gas nozzle 92 through the second gas path 92a, and sprayed onto a target T to thereby form a sprayed coating S on the target T.
In a plasma spraying apparatus in which the wire W is molten into the droplets D by means of the plasma flame F and a second gas flow, and the droplets D are sprayed onto the target T, it is necessary to stably generate plasma flame F, and it is also necessary for a tip end of the wire W to be always disposed within the plasma flame F in order to uniformly spray the droplets D.
However, in the conventional plasma spraying apparatus, the wire guide hole 95 through which the wire W is supplied has a circular cross-section, and is designed to have a greater diameter than that of the wire W in order to prevent the wire W from being hooked or clogged in the wire guide hole 95 due to deformation the wire W originally has. Accordingly, it is difficult to supply the wire W with the distortion of the wire W being reformed, and thus, the wire W repeats going out of and going back to a center of the plasma flame F due to the original deformation of the wire W. Thus, the conventional plasma spraying apparatus is accompanied with a problem that it is not possible to stably supply the wire W to a center of the plasma flame F.
In order to solve the above-mentioned problem, for instance, Japanese Patent Application Publication No. H9 (1997)-308970 has suggested a plasma spraying apparatus including a first guide for reforming original deformation of a wire inserted into a support plate formed integral with a plasma arc torch, and a second guide for guiding the wire from the first guide, and causing the wire W to bend beyond elastic limit thereof. The wire is supplied after the original deformation of the wire was removed, and a tip end of the wire is kept at a center of plasma gas flow to thereby stably generate plasma flame.
However, as suggested in the above-mentioned Publication, if the second guide were integrated with the plasma arc torch for causing a wire to bend beyond elastic limit, a force for feeding a wire would become excessive, because the second guide causes a wire to bend elastic limit thereof. Accordingly, a wire feeding unit is inevitably big-sized, and the torch would be big-sized at its entirety.